Very good article.
There is also the issue of Trademark. Your Pepsi/coke comparison really falls under that category. The "look" is protected when it comes to Trademarks. That is really not the technical definition but is how I look at it in simple terms.
That being said I have seen "art" that uses either old pop bottles or even images of bottles and as far as I know they would not have paid for the right to do that. I think that at some point in a case like that you have changed what it actually is by your creative process thus changing the copyright so to speak.
As you say the moment you create something it is copyrighted but don't forget that you have to prove that.
That is where registering either a copyright or trademark comes into play. It give you legal documentation that you did indeed create it. I know that is a bit off topic for your article but thought it was worth pointing out since you mention it in the first part.
I think it is a great article and helps clarify the issue for many folks. Like you say at the end of your article is is always best to check or get permission.
